1. Field of the Invention
This invention generally relates to a copolymer of hydroxystyrene and a (meth)acrylate wherein some phenolic hydroxyl groups are crosslinked with acid labile groups, and more particularly, to a positive working resist composition, especially of the chemical amplification type, comprising the copolymer as a base resin which has a much improved alkali dissolution contrast before and after exposure, high sensitivity, and high resolution, and is well suited as a micropatterning material for VLSI fabrication.
2. Prior Art
While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. Deep-UV lithography is capable of achieving a minimum feature size of 0.5 .mu.m or less and, when a resist having low light absorption is used, can form patterns with sidewalls that are nearly perpendicular to the substrate.
Recently developed acid-catalyzed chemically amplified positive resists, such as those described in JP-B 2-27660, JP-A 63-27829, U.S. PAT. NO. 4,491,628 and U.S. PAT. NO. 5,310,619, utilize a high-intensity KrF excimer laser as the deep-UV light source. These resists, with their excellent properties such as high sensitivity, high resolution, and good dry etching resistance, are especially promising for deep-UV lithography.
Such chemically amplified positive resists include two-component systems comprising a base resin and a photoacid generator, and three-component systems comprising a base resin, a photoacid generator, and a dissolution inhibitor having acid labile groups.
For example, JP-A 62-115440 describes a resist comprising poly-p-tert-butoxystyrene and a photoacid generator. JP-A 3-223858 describes a similar two-component resist comprising a resin bearing tert-butoxy groups within the molecule, in combination with a photoacid generator. JP-A 4-211258 describes a two-component resist which is comprised of polyhydroxystyrene bearing methyl, isopropyl, tert-butyl, tetrahydropyranyl, or trimethylsilyl groups, together with a photoacid generator. JP-A 6-100488 discloses a resist comprised of a polydihydroxystyrene derivative, such as poly[3,4-bis(2-tetrahydropyranyloxy)styrene], poly[3,4-bis(tert-butoxy-carbonyloxy)styrene] or poly[3,5-bis(2-tetrahydro-pyranyloxy)styrene], and a photoacid generator.
However, when the base resin in these resists bears acid labile groups on side chains and these acid labile groups are groups such as tert-butyl and tert-butoxycarbonyl which are cleaved by strong acids, the resist pattern tends to take on a T-top profile. By contrast, since alkoxyalkyl groups such as ethoxyethyl are cleaved by weak acids, their use has the drawback that the pattern configuration considerably narrows as the time interval between exposure and heat treatment increases. Moreover, the presence of bulky groups on the side chains lowers the heat resistance of the resin and makes it impossible to achieve a satisfactory sensitivity and resolution. These problems have hitherto prevented the practical implementation of either approach, and workable solutions have been sought.
Other known resist compositions use (meth)acrylate copolymers for achieving a higher transparency, improving the adhesion to the substrate and restraining the footing to the substrate as disclosed in JP-A 8-101509 and 8-146610. The resist compositions of this type suffer from low heat resistance and partial pattern collapse.